An automotive steel pipe is required to possess high properties with respect to corrosion resistance and chemical resistance. To provide an automotive steel pipe having enhanced corrosion resistance, it is common practice to cover the surface of a steel pipe with a zinc plating film and cover the plating film with a chromate conversion coating.
Automotive piping generally runs in the bottom of an automotive body and extends to an engine. An automotive steel pipe, therefore, needs to possess high chipping resistance so that a plating film will not be damaged e.g. by a pebble bouncing off the road and hitting the pipe.
FIG. 3 is a cross-sectional diagram of a conventional automotive steel pipe. In the conventional automotive steel pipe, the surface of a steel pipe 1 is coated with a zinc plating film 2 having a thickness of 1 to 100 μm, a chromate conversion coating layer 3 is formed on the zinc plating film 2, and the coating layer 3 is coated via a primer 4 with a resin layer 5 having a thickness of 50 to 300 μm.
Although such a conventional automotive steel pipe has a sufficient corrosion resistance due to the plating film 2, it is weak against impact e.g. when it is hit by a pebble. In the case of an automotive steel pipe for which chipping resistance is required to prevent damage to the plating film, it is necessary to make the outermost resin layer 5 as thick as possible to protect the zinc plating film 2. A fluororesin is most commonly used for the resin layer 5. In conventional practice, a primer, comprising an epoxy resin, is applied to the chromate conversion coating layer 3, followed by baking, and thereafter the resin layer 5 is formed by extrusion.
Instead of the costly fluororesin, a polyamide resin, which is relatively inexpensive and has a high strength, has recently been used for the outermost resin layer of an automotive steel pipe. When a polyamide resin coating is employed, in order to enhance the adhesion of the coating to a primer, an epoxy-based resin having a three-dimensional crosslinking structure is used as a primer, or an adhesion-imparting agent is mixed into a polyamide resin.